This invention pertains generally to selective etching techniques and more particularly to a process for removing silicon dioxide (SiO.sub.2) from a semiconductor substrate.
SiO.sub.2 is widely used in the manufacture of semiconductor devices to control the diffusion of impurities in selected areas on the surface of a silicon substrate. A pattern of SiO.sub.2 is generally formed by first depositing or otherwise forming a layer of SiO.sub.2 on a semiconductor substrate, then forming a mask of positive or negative photoresist on the SiO.sub.2 layer, with windows exposing portions of the SiO.sub.2 which are to be removed to leave the desired pattern. The exposed portions of the SiO.sub.2 are then removed with a suitable etchant.
Etchants heretofore utilized for removing SiO.sub.2, for example, hydrofluoride acid and nitric acid with a hydrofluoric buffer, have not provided satisfactory results from the standpoint of undercutting. Moreover, the acids are in liquid form and their use is therefore subject to the problems commonly attendant to wet chemical processes. There have been attempts to use hydrogen fluoride gas, but the results obtained with this gas have been erratic and unpredictable.
In addition, many finished semiconductor devices have a layer of SiO.sub.2 overlying a silicon substrate, with metallic contacts extending through the SiO.sub.2 layer to the substrate. The formation of contact openings in the SiO.sub.2 layer without damaging the underlying substrate is a delicate process, which has been difficult to conrol with prior art techniques.